The only definitive way to know Plan B worked is getting your period, followed by a negative pregnancy test if your period is late. There’s no immediate signal your body gives you after taking the pill. Plan B works silently by delaying or preventing ovulation, so the confirmation comes in the days and weeks that follow, not in the hours after you swallow it.
What to Watch For in the First Few Weeks
Your period is the clearest sign. If it arrives on time, or even a few days early or late, Plan B almost certainly worked. But “on time” gets tricky because Plan B itself commonly shifts your cycle. Your next period may come earlier or later than expected, and it may look different: heavier, lighter, or more spotty than usual. All of that is normal and doesn’t mean anything went wrong.
If your period hasn’t shown up within three weeks of taking Plan B, take a home pregnancy test. Three weeks is the key threshold because it gives your body enough time to produce detectable levels of pregnancy hormones if conception did occur. Testing earlier than that can give you a false negative, so waiting matters even though it’s stressful.
Side Effects That Mimic Pregnancy
This is where a lot of anxiety comes from. Plan B can cause nausea, fatigue, dizziness, breast tenderness, and spotting. Early pregnancy can cause nausea, fatigue, dizziness, breast tenderness, and spotting. The overlap is almost total, which means you cannot use how you feel as a reliable indicator either way.
Light spotting a few days after taking Plan B is a common side effect of the hormone surge, not implantation bleeding. Nausea that hits within hours of taking the pill is the medication, not morning sickness. But if symptoms like increased urination, persistent nausea, or unusual fatigue continue beyond two or three weeks, a pregnancy test will give you a clearer answer than symptom-checking ever will.
How Effective Plan B Actually Is
Timing is the single biggest factor in whether Plan B works. Taken within the first 24 hours after unprotected sex, it’s roughly 94% effective. By 72 hours (the outer limit of its window), effectiveness drops to about 58%. That’s a steep decline, which is why pharmacists and doctors stress taking it as soon as possible.
Plan B works by delivering a large dose of a synthetic hormone that delays or stops the release of an egg. If ovulation hasn’t happened yet, the pill can prevent it. If you’ve already ovulated, Plan B is far less likely to prevent pregnancy. This is why it’s more effective early in your cycle and less reliable if taken close to or after ovulation.
Body Weight Can Affect How Well It Works
This is something many people don’t hear about. Research shows Plan B’s effectiveness drops significantly at higher body weights. For people weighing under about 165 pounds (75 kg), the estimated pregnancy rate after taking Plan B stays below 2%. Between 165 and 187 pounds (75 to 85 kg), that rate jumps to roughly 6%, which is close to the pregnancy risk of using no contraception at all. The sharpest decline in effectiveness happens in the 155 to 175 pound range.
If you weigh more than 165 pounds and are concerned about effectiveness, a copper IUD inserted within five days of unprotected sex is the most effective form of emergency contraception regardless of weight. Another prescription option, ella (ulipristal acetate), maintains better effectiveness at higher body weights than Plan B does, though it requires a prescription or telehealth visit.
Medications That Can Reduce Effectiveness
Certain drugs can interfere with how your body processes Plan B. Anti-seizure medications like phenytoin, carbamazepine, and topiramate speed up the breakdown of hormones in your liver, potentially reducing the pill’s effectiveness. The tuberculosis antibiotic rifampin has the same effect. If you take any of these regularly, Plan B may not work as well for you, and a copper IUD would be a more reliable emergency option.
What a Positive Test Means
If you take a pregnancy test at the three-week mark and it’s positive, Plan B did not prevent pregnancy. This doesn’t mean you did anything wrong. No emergency contraception method is 100% effective, and the pill cannot work if ovulation has already occurred. A positive result means it’s time to schedule an appointment with a healthcare provider to confirm the pregnancy and discuss your options.
If your test is negative at three weeks but your period still hasn’t arrived, wait another week and test again. Plan B can delay your period by up to a week, and stress can push it even further. A second negative test at four weeks is a strong confirmation that you’re not pregnant and that the cycle disruption is just a temporary side effect of the medication.